Wet spinning process



March 15, 1966 R. o. DENYES WET SPINNING PROCESS Filed Dec. 6, 1962 United States Patent Ofi ice 3,240,852 Patented Mar. 15, 1966 3,240,852 WET SPINNING PROCES Russell Gwens Denyes, Maplewood, NJ, assignor to Celanese Corporation of America, New York, NBA, a corporation of Delaware Filed Dec. 6, 1952, Ser. No. 242,748 6 Claims. (Cl. 26418l) This invention relates to the wet spinning of solutions of filament-forming materials and more particularly to the formation of filaments of cellulose triacetate characterized by a reduction in the number of filaments which are cemented together to form undesirable splinters.

Synthetic filaments are prepared by forcing a filamentforming material in fluid condition through apertures in a device known as a spinnerette. The filament-forming material may be forced through the spinnerette as a melt solidifying by cooling; it may be discharged as a solution or dope into an evaporative medium which removes the solvent; or it may be discharged as a solution or dope into a liquid coagulating medium which is miscible with the solvent for the filament-forming material itself.

Spinning into a liquid coagulating medium known as Wet spinning, can be effected by continuously passing the dope from the spinnerette into and through a column or tube through which the coagulating medium is continuously flowing. The filaments are pulled through the coagulating medium and generally are drawn down, i.e., the linear speed at which the filaments are withdrawn from the spinning column is greater than the linear speed at which the filament-forming material is discharged through the spinnerette apertures.

The total number of filaments spun from a single spinnerette is frequently considerable. For example, spinnerettes having over 1,000 individual filament-forming orifices are not uncommon. Of course, the greater the number of filaments spun from a single spinnerette, the greater the opportunity for adjacent filaments to contact one another. Moreover, unless the interior filaments of the filament bundle can be reached by the coagulating medium, there is insufficient coagulation of the interior filaments as they pass through the spinning column. Accordingly, when such insufficiently-coagulated filaments pass out of the spinning column and contact other filaments making up the filament bundle, there is caused a marrying or cementing together of such filaments, leading to the formation of splinters. (In this application, the expression married designates filaments which adhere to one another but which can be pulled apart; cemented refers to filaments which are permanently adhered to one another and cannot be peeled apart; wiry refers to exceptionally high denier filaments, the high denier generally resulting from the fact that such filaments are not drawn along with the remainder of the filament bundle.)

Cementing together of filaments and consequent splinter formation is also associated with the presence in the spinning column of local high concentrations of solvent. When a partially coagulated filament contacts a zone of such local high concentration, the relatively concentrated solvent in such zone tends to re-dissolve the filament and counter the effectiveness of the coagulating medium, whereupon when the filament leaves the spinning column, it has not been sumciently coagulated. Thus, any such insufficiently coagulated filament, when brought into contact with another filament in the filament bundle, will tend to cement itself thereto, again leading to the production of undesirable splinters.

It is therefore an object of the present invention to provide a process whereby the number of filaments which are cemented together is markedly reduced.

Another object of the invention is to provide a wet spinning process whereby the number of splinters in the filaments is materially reduced.

Additional objects and advantages will become apparent hereinafter.

In accordance with the present invention, a dope of filament-forming material is wet spun into a spin bath which flows through a zone, hereinafter called a high liquid velocity zone, of the column in the same direction as, and at a linear velocity greater than, that of the filament-forming material. This zone of rapid linear spin bath flow (high liquid velocity zone) is followed by a second zone, hereinafter called a low liquid velocity zone, in which zone the spin bath is flowing more slowly than the filaments are travelling. This low liquid velocity zone is followed by a second high liquid velocity zone (wherein the linear velocity of the spin bath is greater than that of the filaments), which in turn is followed by a second zone of low liquid velocity (wherein the spin bath is flowing more slowly than the filaments are travelling). Thereafter, the filaments may be withdrawn from the spinning column. If desired, however, prior to the passage of the filaments from the spinning column additional high and low velocity zones can be provided so that the filaments pass through such additional zones prior to their exit from the spinning column.

A particularly convenient means for providing successive zones of relatively high and relatively low spin bath velocity in the spinning column is by successively reducing and enlarging the cross-sectional area of the spinning column. Since the volume rate of flow of spin bath through the column remains substantially constant, provision of a constricted zone in the column, which zone has a reduced cross-sectional area, results in an increased linear velocity of the spin bath in such zone. If such zone of increased velocity is succeeded by an expansion zone wherein the cross-sectional area is increased, the linear velocity of the spin bath is correspondingly decreased.

Successive changes in the linear velocity of the spin bath are conveniently effected by gradually decreasing, then gradually increasing, then gradually decreasing, etc., the cross-sectional area of the spinning column. I have found that best results are obtained when the crosssectional area of each expansion zone is at least 25% and preferably at least 50% greater than the cross-sectional area of each constriction zone. Such increase in cross-sectional area in the expansion Zone will decrease the linear velocity of the spin bath through such expansion Zone by at least 20%. More preferably, such linear velocity will be decreased by at least 33 Although the precise mechanism by which the superior results of the present invention are obtained is not known with certainty, such improved results, as regards reduction in the number of filaments which are cemented together, are due to the presence of alternate constriction and expansion zones in the spinning column. Thus, in any given constriction zone, the linear velocity of the spin bath is greater than that of the filament-forming material or filament, so that the spin bath effects some attenuation of the partially coagulated filaments due to the local speed differential. Conversely, in any given expansion zone, the linear velocity of the spin bath is slower than the speed with which the filaments are travelling, so that again a local speed differential is set up. However, in a constriction zone the local speed differential is such that the filaments exert a drag on the spin bath, whereas in any given expansion zone the spin bath exerts a drag on the filaments. It appears that the successive changes in spin bath velocity as the spin bath passes from constriction zone to expansion zone to constriction zone, etc., with the consequent changes in drag, results in far better penetration by the spin bath into the interior of the filament bundle. Further such relative changes in spin bath velocity tend to disperse any local zones of high solvent concentration, which local zones would otherwise tend to re-dissolve partially coagulated filaments. Accordingly, there is more complete coagulation of interior filaments, so that when the filament bundle is withdrawn from the spinning column, the tendency of any interior filaments to cement themselves together is greatly reduced.

It is desirable that the spin bath, i.e., coagulating liquid, exert a swelling action on the filament-forming material, whereby the initially extruded filament-forming material is partially coagulated and somewhat swollen. Such partial swelling of the partially coagulated filaments prevents too rapid a coagulation thereof and hence permits an attenuation of the filaments in the first constriction zone, thereby improving the physical properties of such filaments.

In US. Patent 3,071,806, there is described a spinning column with a single constriction, which column is used to form filaments having superior physical properties. However, as will be demonstrated hereinafter, the use of a single constriction spinning column, while helpful in improving the physical properties of wet-spun filaments, is not overly effective as regards prevention of splinter formation. Moreover, while it might be expected that, if a single constriction resulted in improved physicals for the spun filaments, e.g., improved tensiles, then the use of a spinning column with multiple constrictions would further improve the physicals of the filaments. In fact, such use of a multiple constriction column does not appear to further enhance the physical properties of the filaments. Surprisingly, however, the use of such a multiple constriction column does markedly reduce the amount of fiaments which are cemented together, and hence the number of splinters in the filaments, and does so without substantially altering the physical properties of the filaments.

The novel process of wet spinning filament-forming materials can be effected with dopes of cellulose derivatives such as cellulose acetate, cellulose propionate, cellulose butyrate, cellulose acetate-propionate, cellulose benzoate, methyl cellulose, ethyl cellulose, benzyl cellulose, and the like. Particularly desirable results are realized when using solutions of cellulose esters containing less than 0.29 free hydroxyl groups per anhydroglucose unit (hereinafter referred to as cellulose triesters) such as cellulose triacetate of acetyl value of at least 59% and preferably at least 61% by weight in methylene chloride, advantageously blended with up to about 15% by weight of the total solvent of a lower alkanol such as methanol, ethanol or propanol, of which methanol is preferred. The presence of a small amount of water in the solvent, e.g., about 1% by weight, will not interfere with the process.

For the sake of brevity, the invention will be further described with reference to the spinning of a solution of cellulose triacetate dissolved in a mixture of methylene chloride and methanol. The cellulose triacetate preferably has an intrinsic viscosity in the range of 1.5 to 3, best results being obtained when the intrinsic viscosity is at least about 2. The intrinsic viscosity referred to above is that of the regenerated cellulose obtained by complete saponification, without degradation, of the cellulose triacetate. It can be determined according to wellknoWn accepted procedures, using a solution of the regenerated cellulose in cupriethylene-diamine.

The cellulose triacetate dope is spun into a coagulant therefor, which is preferably a mixture of methylene chloride and methanol wherein the methylene chloride weight percent is preferably within the range of 75 Ti-5 where T is the temperature of the spin bath in C. This temperature preferably ranges from about 20 to 50 C. so that the preferred methylene chloride content of the spin bath ranges from about 20 to 60% by weight.

The spinning column may be disposed horizontally or on an incline but is preferably directed vertically with the dope and spin bath moving upwardly. The linear speed of the extrusion of dope can be varied widely, ranging generally from about 5 to 50 meters per minute. The filaments are taken up at a linear speed ranging from about 30 t0 3 times the extrusion speed and preferably from about 10 to 5 times the extrusion speed. The highest take-up speed consistent with superior physical properties and freedom from filament discontinuities is preferred since the capacity of the equipment is thereby increased.

The spin bath is fed to the column at such a volumetric rate that its linear rate of flow in the high liquid velocity zones ranges from about 40 to 90% and preferably 50 to of the filament take-up speed. Advantageously, the spin bath speed in the high liquid velocity zone is at least twice and preferably at least three times the extrusion speed. As the partially coagulated filaments pass through the initial high liquid velocity zone they are rapidly attenuated by the fast moving spin bath and are speeded up. At the same time they are oriented in a manner which will produce high tenacity upon completion of coagulation. The length of the high speed and low speed zones are, of course, selected to give optimum physical properties, consistent with minimum splinter formation.

For a spinning column having an overall length of 40 inches, the length of the initial constriction (high liquid velocity) zone should be about 1 to 6 inches and preferably about 3 /2 inches. Similarly, the length of the initial expansion (low liquid velocity) zone should be about 3 inches. The length of the next high liquid velocity zone should be about 1 to 6 inches, and more preferably about 3%. inches. The length of the next low liquid velocity zone should be about 10 to 50 inches, and preferably about 30 inches.

As previously indicated, while the benefits of the instant invention are secured only if at least two constriction zones are provided in the column, additional constriction zones may also be provided. In such case, best results are obtained when the constriction zones are of approximately equal length and when the expansion zones are also of approximately equal length. The sum of all of the lengths of the constriction zones is desirably from about 15 to 50% of the total length of the column. The sum of all of the lengths of the expansion zones is correspondingly desirably from about to 50% of the length of the column. Best results are obtained when a given constriction zone communicates with a given expansion zone by means of a gradual taper, which taper should generally not exceed about 45 degrees. In practice, this means that the length of a given taper section connecting a given constriction zone with a given expansion zone is from about 5 to 50% of the length of such constriction zone.

In order to secure the optimum benefits of the present invention, the initial constriction zone should commence at a distance of from about 1 to 10 inches from the spinnerette face.

Beyond the initial constriction (high liquid velocity) zone, the linear rate of flow of the spin bath is gradually diminished so that in the first expansion (low liquid velocity) zone, the filaments will be moving at a linear speed greater than that of the spin bath, generally at least 5 meters per minute greater. This differential creates a hydraulic drag as described previously.

The change in cross-section between the constriction zones and expansion zones is best effected by providing tapered transitional sections in the spinning column, each of which tapered section communicates with a constriction zone and an expansion zone. The tapered section is generally produced by providing an inclined shoulder in the column, whereby the intermediate tapered transition zone is established.

The linear velocity of the filaments varies along the length of the spinning column. The initial and final speeds, i.e., extrusion and take-up speeds are set when starting operation. The linear speed varies inversely with the cross-section of the filaments and thus determination of the filament speed at intermediate locations can be made in several ways by measuring the cross-sectional area of the filaments. This measurement can be made by light transmission since the transmission in a plane perpendicular to the column will depend upon the area of that plane occupied by cellulose triacetate and the area occupied by spin bath. Color may be added substantive either to the spin bath or cellulose triacetate to broaden the differences in light transmission.

Another procedure for measuring the cross-sectional area of the filaments at difi'erent locations along the spinning column, and thus of measuring the filament speed, involves terminating extrusion and take-up of filamentary material at the same time, whereby there will be formed a filament rope free at its lower end. The filament rope is permitted to remain in the spin bath for several seconds to harden, and is then removed and dried with little or no shrinkage. The dried filaments are then cut transversely and the cross-sectional area at various locations is measured.

The invention will now be described more fully with reference to the accompanying drawing, schematically showing partly in elevation and partly in section an apparatus suited for practicing the invention.

Referring to the drawing, there is shown a spin pot housing a spinnerette 12 for the extrusion of dope which is supplied thereto to a conduit 14. The spin pot 10 is provided with an inlet 16 for the admission of spin bath, i.e., coagulating liquid. The spin pot 10 communicates directly with a spinning column generally designated by the reference character 18, which column is provided near its open top with a liquid runoif 20 to carry away spin bath. The column is provided with a tapered shoulder 22 which serves as an intermediate or transitional zone leading into initial constriction zone 24. Zone 24 communicates with tapered shoulder 26, which shoulder terminates in initial expansion zone 28. Similarly, zone 28 communicates with tapered shoulder 30, which shoulder leads into second constriction zone 32, which constriction zone communicates with second expansion zone 34 via tapered shoulder 36. Dope spun through spinnerette 12 forms a plurality of filaments 38 which pass upwardly through constriction zone 24, expansion zone 28, constriction zone 32, expansion zone 34, and then through the balance of column 18 to ultimately pass out of the column and about a positively driven feed roll 40 and idler 42. Thereafter, the filament can be washed, dried, stretched, or subjected to any other desired aftertreatment.

The following example will further illustrate the invention.

EXAMPLE A spinning apparatus was constructed as shown in the drawing with a spinning column length of 50 inches. The face of the spinnerette was positioned at a distance of 2 inches below the spinning column. The spinning column, for its first 1% inches of length, had a diameter of one inch, then tapered inwardly (taper angle equals 9") to a diameter of of an inch. The length of this transitional section was A of an inch. The length of the inch diameter constriction zone was 3 /2 inches. At the end of this zone the column tapered outwardly to a diameter of one inch to provide an expansion zone, which expansion zone was 3% inches in length. This expansion zone then tapered inwardly to a constriction zone of 4 inch diameter, the length of the constriction zone being 3% inches. The constriction zone then tapered outwardly to an expansion zone of one inch diameter, which zone extended throughout the remaining length of the column (35% inches). The spinnerette was provided with 4,060 holes, each 0.100 mm. in diameter. Three spin baths of varying amounts of methylene chloride and methanol were passed through the pot and column at a rate of 20 liters per minute at a temperature of 35 C.

Cellulose triacetate having an acetyl value of 61.5%, calculated as acetic acid, and an intrinsic viscosity of 2.0 as measured in cupriethylenediamine on cellulose regenerated therefrom, was dissolved in 91/9 weight mixture of methylene chloride/methanol to form 21.8 weight percent solution. The dope was extruded through the spinnerette at a linear velocity of 7 meters per minute and the feed roll was run at meters per minute. The speed of the spin bath through the first constriction zone was about 70 meters per minute; the speed through the first expansion zone, about 39 meters per minute; the speed through the second constriction zone about 70 meters per minute, and the speed through the second expansion zone about 39 meters per minute.

The filaments, on leaving the column, were withdrawn at the speed of the feed roll, i.e., 80 meters per minute. Following withdrawal from the column, they were dried and hand cut into two-inch staple lengths. Samples were tested for fiber properties and for splinter count.

In order to compare the effect of a column having two constriction zones as compared to a column having only one, the above procedure was utilized. However, the column used contained only the first constriction zone, namely zone 24. Thus, zone 24 terminated at tapered shoulder 26, which in turn terminated at expansion zone 23, and the expansion zone 28 extended throughout the remainder of the length of the spinning column.

The reduction in splinter count when using a column having at least two constriction zones is readily apparent from the data. The physical properties of the filaments obtained using a double constriction column were substantially the same as those of the filaments obtained using a single constriction column.

Variations can, of course, be made without departing from the spirit of the invention.

Having thus described the invention, what is desired to be secured and claimed by Letters Patent is the following:

1. A process for wet-spinning a solution of a filamentforming material whereby the amount of splinters in the resulting filaments is markedly reduced, which process comprises extruding said solution at a linear velocity of about 5 to 50 meters per minute into a spin bath exerting a swelling action thereon to form a partially .coagulated swollen filamentary material, passing said spin bath and partially coagulated filamentary material through a first zone in which said spin bath travels at a greater linear speed than the linear speed of said partially coagulated filamentary material, passing said spin bath and said partially coagulated filamentary material through a second zone in which said spin bath travels at a linear speed less than that of said filamentary material, passing said spin bath and said filamentary material through a third zone in which said spin bath travels at a greater linear speed than that of said filamentary material, passing said spin bath and filamentary material through a fourth zone in which said spin bath travels at a linear speed less than that of said filamentary materal, and withdrawing said filamentary material from said column at a speed ranging from about 3 to 30 times the extrusion speed, said spin bath travelling substantially parallel to said filamentary material in said zones, the linear velocity of said spin bath in said first and third zones being about 40 to 90% of said withdrawal speed and at least twice said linear velocity of extrusion, the linear speed of the filamentary material being at least meters per minute greater than the linear velocity of the spin bath in said second and fourth zones.

2. A process for wet-spinning a solution of a filamentforming material whereby the amount of splinters in the resulting filaments is markedly reduced, which process comprises extruding said solution at a linear velocity of about 5 to 50 meters per minute into a spin bath exerting a swelling action thereon to form a partially coagulated swollen filamentary material, passing said spin bath and partially coagulated filamentary material through a first constriction zone in which said spin bath 'travels at a greater linear speed than the linear speed of said partially coagulated filamentary material, said constriction zone commencing at a maximum distance of about 10 inches from the point of extrusion, passing said spin bath and said partially coagulated filamentary material through a first expansion zone in which said spin bath travels at a linear speed less than that of said filamentary material, passing said spin bath and said filamentary material through a second constriction zone in which said spin bath travels at a greater linear speed than that of said filamentary material, passing said spin bath and filamentary material through a second expansion zone in which said spin bath travels at a linear speed less than that of said filamentary material, and withdrawing said filamentary material from said column at a speed ranging from about 3 to times the extrusion speed, said spin bath travelling substantially parallel to said filamentary material in said zones, the linear velocity of said spin bath in said first and third zones being about to 90% of said withdrawal speed and at least twice said linear velocity of extrusion, the linear speed of the filamentary material being at least 5 meters per minute greater than the linear velocity of the spin bath in said second and fourth zones.

3. The method of claim 1 wherein said filament-forming material comprises a solution of a cellulose ester containing less than 0.29 free hydroxyl groups per anhydroglucose unit.

4. The method of claim 3 wherein the solvent for said cellulose ester is a mixture of methylene chloride and a lower alkanol.

5. The method of claim 4 wherein said cellulose ester is cellulose triacetate.

6. The method of claim 3 wherein said spin bath is at a temperature of from about 20 to C. and comprises a mixture of methylene chrloride and an alkanol, the amount of methylene chloride in said bath being from about 20 to percent by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,027,419 1/1936 Dreyfus 264l81 2,732,586 1/1956 Bradshaw et a1. 18-8 2,983,952 5/1961 Edwards et a1. l88 3,049,755 8/1962 Aizawa et al. 18-8 3,071,806 1/1963 Rosenthal 264181 ALEXANDER H. BRODMERKEL, Primary Examiner.

MORRIS LIEBMAN, Examiner. 

1. A PROCESS FOR WET-SPINNING A SOLUTION OF A FILAMENTFORMING MATERIAL WHEREBY THE AMOUNT OF SPLINTERS IN THE RESULTING FILAMENTS IS MARKEDLY REDUCED, WHICH PROCESS COMPRISES EXTRUDING SAID SOLUTION AT A LINEAR VELOCITY OF ABOUT 5 TO 50 METERS PER MINUTE INTO A SPIN BATH EXERTING A SWELLING ACTION THEREON TO FORM A PARTIALLY COAGULATED SWOLLEN FILAMENTARY MATERIAL, PASSING SAID SPIN BATH AND PARTIALLY COAGULATED FILAMENTARY MATERIAL THROUGH A FIRST ZONE IN WHICH SAID SPIN BATH TRAVELS AT A GREATER LINEAR SPEED THAN THE LINEAR SPEED OF SAID PARTIALLY COAGULATED FILAMENTARY MATERIAL, PASSING SAID SPIN BATH AND SAID PARTIALLY COAGULATED FILAMENTARY MATERIAL THROUGH A SECOND ZONE IN WHICH SAID SPIN BATH TRAVELS AT A LINEAR SPEED LESS THAN THAT OF SAID FILAMENTARY MATERIAL, PASSING SAID SPIN BATH AND SAID FILAMENTARY MATERIAL THROUGH A THIRD ZONE IN WHICH SAID SPIN BATH TRAVELS AT A GREATER LINEAR SPEED THAN THAT OF SAID FILAMENTARY MATERIAL, PASSING SAID SPIN BATH AND FILAMENTARY MATERIAL THROUGH A FOURTH ZONE IN WHICH SAID SPIN BATH TRAVELS AT A LINEAR SPEED LESS THAN THAT OF SAID FILAMENTARY MATERIAL, AND WITHDRAWING SAID FILAMENTARY MATERIAL FROM SAID COLUMN AT A SPEED RANGING FROM ABOUT 3 TO 30 TIMES THE EXTRUSION SPEED, SAID SPIN BATH TRAVELLING SUBSTANTIALLY PARALLEL TO SAID FILAMENTARY MATERIAL IN SAID ZONES, THE LINEAR VELOCITY OF SAID SPIN BATH IN SAID FIRST AND THIRD ZONES BEING ABOUT 40 TO 90% OF SAID WITHDRAWAL SPEED AND AT LEAST TWICE SAID LINEAR VELOCITY OF EXTRUSION, THE LINEAR SPEED OF THE FILAMENTARY MATERIAL BEING AT LEAST 5 METERS PER MINUTE GREATER THAN THE LINERAR VELOCITY OF THE PSIN BATH IN SAID SECOND AND FOURTH ZONES. 